Field of the Invention
This invention relates to a method and apparatus for cooling rolled steels, and more particularly to a method and apparatus for cooling rolled steels whereby excellent low-temperature toughness is obtained by rapidly cooling and hardening just-rolled high-temperature steels in cooling water and thus improving their surface grain structure.
Description of the Prior Art
A method disclosed in Japanese Provisional Patent Publication No. 114638 of 1982 is an example of several known methods of producing roller steels having excellent low-temperature toughness by directly improving their surface grain structure (through the application of surface hardening). According to this method, the surface area of steel product hot-rolled to a given diameter or cross-sectional size is successively cooled with water spray from above the Ar.sub.1 transformation temperature to below the bainite transformation temperature, or preferably from above the Ar.sub.3 transformation temperature. Namely, the cooling is effected so that the ratio of the heat-transfer rate .alpha..sub.s at the surface of the rolled steel to the heat-transfer rate .sub.i across the radius thereof is .alpha..sub.s &gt;.alpha..sub.i. It is known that the surface portion of the rolled steel thus rapidly cooled and from which the water is then removed by means of high-pressure air blown thereagainst and which is thus further cooled becomes hot again the temperature becomes reelevated to a reelevated temperature, because of the heat transferred from the hotter core while being conveyed in the atmosphere from the water-cooling apparatus to cooling beds or a coiler (as disclosed in Japanese Provisional Patent Publications Nos. 134513 of 1974, 90912 of 1976 and 99619 of 1976).
As was proposed n Japanese Patent Publication No. 48566 of 1981, the water-cooling apparatus comprises a plurality of cooling units (cooling boxes) that are disposed in tandem. Each cooling box has annular forward- or backward spraying nozzles from which high-pressure cooling water is ejected against the rolled steel. The aforementioned water-removing apparatus is provided midway in the series of cooling boxes.
To produce products having the desired mechanical properties, the cooling rate and time must be controlled in accordance with the diameter or cross-sectional size of rolled steels. For this purpose, the surface temperature of the rolled steel has conventionally been determined at a point considerably far away from the exit end of the cooling apparatus so that the cooling rate could be controlled through the adjustment of the amount or pressure of the cooling water in accordance with the determined temperature.
In the conventional method just described, the recovered temperature measured at a point considerably far away from the exit end of the cooling apparatus has been used as the basis for controlling the amount or pressure of the cooling water. As such, when a problem arose, it has been difficult to quickly apply a corrective action to the following portion of the same piece or to the next piece. It takes approximately 10 to 60 seconds for the rapidly cooled piece to reach the reelevated temperature measuring point, so application of a corrective action within the same piece has been delayed by the same length of time. Also, a similar delay has occurred when the delivery interval between the defective piece and the next piece was shorter than the above time that is needed in reaching the reelevated temperature measuring point. Because of this shortcoming, the approximate length of the rapidly cooled section and the required amount of cooling water have had to be predetermined using a test material. Not only has such a testing procedure has been non-productive, but also the used test material has had to be discarded as scrap.
With the conventional method just described, the cooling rate and end-point temperature in quenching have had to be estimated by simulation or other similar technique on the basis of the reelevated surface temperature at a point considerably far away from the exit end of the cooling apparatus. Here, the end-point temperature of quenching means the surface temperature that is reached when the rolled piece has been cooled to a given temperature from the surface to a desired depth thereunder. Meanwhile, the cooling rate varies intricately with other operating conditions, so it is difficult to exactly tell whether quenching to the desired depth had been achieved only on the basis of the temperature measured on the exit side of the cooling apparatus. Accordingly, it has been difficult to perform uniform quenching at the desired cooling rate that is essential to steady production of steel products having the desired mechanical properties. Simulation has had to be done over again every time operating conditions changed, resulting in inefficient operation control.
In the rolling of bars and wire rods, various rolling conditions, such as their diameter, rolling speed or the length of time in which they pass through a cooling apparatus, are varied from time to time. For the production of satisfactory products, it is essential to provide an optimum cooling apparatus that functions appropriately with varying conditions. Particularly, the production of rolled steel for low-temperature services having excellent low-temperature toughness calls for extremely close control. Therefore, the temperature with which the piece leaves the finishing stand, the temperature at which quenching is finished and/or the reelevated temperature of the piece must be controlled at appropriate levels.
If the cooling area is longer than necessary, for instance, the cooling time becomes too long. Then, if the reelevated temperature is kept within the desired range, the quenching ending temperature becomes so high that the desired limit is exceeded. If, conversely, the quenching ending temperature is kept within the desired range, the reelevated temperature becomes too low. In all such cases, satisfactory products cannot be obtained. If the cooling area is too short, the results are reversed. Neither the quenching ending temperature nor the reelevated temperature can be brought in the desired range through the control of cooling water volume or pressure alone. Conceivably, adjustment of the rolling speed or cooling area length will offer a solution to this problem. But the rolling speed cannot be varied freely because of the limitation on mill load and other factors. In contrast, the cooling area length can be appropriately chosen with relative ease. Conventionally, the cooling area length has been empirically determined on the basis of the operational data of the past, for want of any other appropriate measures to cope with varying rolling conditions. When the rolling speed was varied because of the need to roll various sizes of products or of the limited rolling mill capacity, however, quick response has been difficult to achieve. Consequently, it has been difficult to steadily attain the desired properties.